MATLAB: How should be change ROOT LOCUS asymmetric

Control System ToolboxMATLABroot locus

Task is find stabil interval. (root locus)
Code:
num=[1 0 0 0]
den=[1 0 5 10 1.5]
Hs=tf(num,den)
rlocus(Hs)
Result:
num =
1 0 0 0
den =
Columns 1 through 3
1.0000 0 5.0000
Columns 4 through 5
10.0000 1.5000
Transfer function:
s^3
------------------------
s^4 + 5 s^2 + 10 s + 1.5
Plot is asymmetric: http://i.imgur.com/v8dQD.png
Who can me help to solve symetrical plot?

Best Answer

k=-1000:0.1:1000;
rlocus(Hs,k)